1. Field of the Invention
The present invention relates generally to an exhaust emission purifying apparatus for reductively purifying nitrogen oxides (NOx), and in particular, to a technology for increasing a detection accuracy of a concentration meter which is configured to detect the concentration of a liquid reducing agent based on a heat transmission characteristic between two spaced points while accelerating the defrosting of the liquid reducing agent stored in a reducing agent container in the exhaust emission purifying apparatus for reductively purifying NOx.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2000-27627 discloses an exhaust emission purifying apparatus as one proposal for a catalytic converter purifying system for eliminating NOx contained in the exhaust gas from an engine. In such an exhaust emission purifying apparatus, a liquid reducing agent is injection-supplied to the exhaust emission stream in an upstream portion of a reduction catalytic converter disposed in an engine exhaust emission system, depending on engine operating conditions, so that NOx in the exhaust gas and the liquid reducing agent are subjected to the catalytic-reduction reaction, to thereby purify NOx into harmless components.
However, in a cold region, such as Hokkaido Island in Japan, sometimes in winter, the outside temperature falls down to or below a freezing point of a liquid reducing agent, and then, the liquid reducing agent stored in a reducing agent container can be frozen. The liquid reducing agent ordinarily begins to be frozen from a peripheral portion in the container which is in contact with the outside air, and the freezing thereof gradually progresses toward the liquid reducing agent in a container center portion. On a bottom portion of the reducing agent container, there are positioned a detecting section of a concentration meter configured to detect the concentration of the liquid reducing agent based on a heat transmission characteristic between two spaced points, a suction port configured to suck the liquid reducing agent therethrough and the like. Therefore, if the liquid reducing agent is frozen even in slightly, it becomes difficult to perform the detection of the concentration of the liquid reducing agent and the supply thereof. On the reducing agent container, there is mounted a heat exchanger which permits the engine coolant to circulate therein to perform the heat exchange with the liquid reducing agent. However, when a stopping of operation of an engine continues for a long time, since the coolant temperature is low immediately after starting of the engine operation, it necessarily takes a certain time period to defrost the frozen liquid reducing agent.
Therefore, to defrost the liquid reducing agent in a short time after the engine operation is started, it can be considered to provide with a box-like encasing member configured to trap therein the heat discharged from the heat exchanger, which encasing member is disposed around the suction port that sucks the liquid reducing agent and the detecting section of the concentration meter of the liquid reducing agent. However, there is a possibility of the following problem. Namely, if a space or a gap exists between the concentration meter and the encasing member, when the liquid reducing agent is replenished into the reducing agent container, bubbles may sometimes enter the inside of the encasing member together with the liquid reducing agent and adhere to the detecting section of the concentration meter. If bubbles adhere to the detecting section of the concentration meter, the heat transmission characteristic between the two spaced points is changed, resulting in significant degradation of the concentration detection accuracy of the concentration meter. Incidentally, since the concentration meter needs the periodic inspection and maintenance, it is impossible to couple the concentration meter with the encasing member without any space.